1. Field of Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device using lithography. Specifically, the present invention relates to the alignment of a substrate or wafer in the path of a patterned radiation beam.
2. Description of Related Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The scanning of the pattern may follow what is known as a “meander”, which is a possible path taken by the scanning stage to ensure that the entire target portion is included in the scan and is therefore exposed.
In order for the correct portion of the substrate to be exposed, the mask and the substrate must always be aligned. In other words, the accuracy of the product position on the substrate as described above is at least in part determined by the relative position of the substrate and the patterning device during exposure of the pattern. In particular, the accuracy (or lack thereof) of a position of an image of the pattern on the substrate at the moment of exposure is dependent on the relative positional error of the substrate support and the patterning device support at the very instant a radiation beam is transmitted through the patterning device onto the substrate surface. Position errors impact overlay (i.e. the accuracy of one exposed layer on another) and critical dimension uniformity (i.e. the width of product structures on the substrate surface).
However, there are always alignment errors because the movements of the mask (table) and the substrate (support) necessarily move more slowly than the exposure radiation. The supports (or tables) that support the substrate and the patterning device respectively are controlled such that they move with minimum positional error and with constant velocity. In the meantime, the radiation (which is often in the form of a laser) fires pulses at a constant frequency (pulse repetition rate). Accidental or unintentional movement in any of the mechanical devices may occur just at the timing of the radiation and although the radiation beam (or laser) may be in the correct position, the mask and substrate may not, at that precise moment, be perfectly aligned.
There is an increasing requirement for smaller and more precise patterns to be exposed on the substrate. As products become smaller, relative movements of the substrate table and the patterning device table become less tolerable and a cause of errors. In other words, it becomes increasingly difficult to keep the support positional errors below decreasing tolerances. One reason for this is that the supports (or tables) are mechanical components that do not allow instantaneous repositioning or very rapid movements. In addition, disturbance forces like acoustics or noise in electronic components limit the obtainable accuracy. However, the support that supports the patterning device and the support that supports the substrate both need to be aligned during the radiation pulse (i.e. during the exposure of the pattern on the substrate surface) and it is in this alignment that accuracy errors may occur. These errors are often caused by unintentional motion of one of the supports in response to acoustical and/or mechanical vibrations by electronic sources.